DESCRIPTION (Applicant's Abstract): This project would continue the research on human concepts and their representation in the mind. The work investigates two main issues. The first is how the structure of a category interacts with what people know about the category to jointly determine whether the category can be learned. The proposed experiments manipulate the relation between the structure and the knowledge in order to specify a) what the representation of a category is when knowledge is involved in the learning, and b) whether knowledge helps or hurts the extraction of statistical information about a category. The experiments use a standard concept-learning methodology, with the addition that knowledge of the category's domain is experimentally manipulated. The second major issue is the relation between word meanings and concepts, as examined through studies of polysemy, the phenomenon that most words have many related meanings. Polysemy poses a puzzle to any theory of word meanings, because the theory must explain how each separate meaning is learned and represented and what the constraints on multiple meanings are. According to the present theory, word meanings are represented by relations between word forms and concepts; therefore, conceptual similarity constrains the extension of word meanings. The proposed research is the first major experimental investigation into how word meanings are extended to similar senses. The proposed research investigates one of the most basic of all our thought processes--how we perceive and think about objects and events in our world. This is a central topic in the study of the mind, and it is especially relevant to understanding language acquisition, production and disturbances. As our concepts play a central role in reasoning, social cognition, and problem-solving, this work also adds to our understanding of higher-level thought and its disruption in brain damage and mental illness.